There's a battle being waged in the treetops high above Columbia County, a contest of brutal biological warfare, and there is little you can do about it.

In fact, you probably can't even see it.

This tiny battle royale is a natural drama that unfolds every few years as hordes of western tent caterpillars are attacked by an air force of tiny parasitic wasps and other natural enemies.

This summer in the hardwood forests hugging the Columbia River, aerial surveys have detected 13,000 acres of defoliation due to the caterpillar, the larval form of the native Northwest moth Malacosoma californicum. The last major outbreak in the area occurred a dozen years ago.

The caterpillars emerge from eggs in spring, create silken tents in the branches of hardwood trees and begin to eat. And eat. And eat.

While the damage to foliage can be considerable, and their tents can be an eyesore, western tent caterpillars cause little long-term harm to the trees. They also pose no significant risk to human health or to pets.

After 30 to 40 days, the caterpillars leave their tents to wrap themselves in silk cocoons where they pupate. It is at this stage that people are most likely to encounter the pests directly.

"Walking in the forest, you can hear a light pitter-patter like rain. That is the sound of caterpillars dropping from the trees," says Amy Grotta, forestry and natural resource agent for the OSU Extension in Columbia County. "You can't take a step without getting them on your clothes or stepping on them."

If control is necessary, the Oregon Department of Forestry recommends non-chemical treatments, such as high-pressure water spray or pruning. Burning the silk tents is a bad idea; it's likely to cause greater long-term damage to the tree.

Grotta emphasized that, at this point in the life cycle of the pest, it is best to simply let the infestation run its course and leave the viruses and tiny wasps to wage their tiny war. "Once you get enough caterpillars, the population of natural enemies builds up. After the caterpillar population crashes, it takes a long time to build up again."

"For me as a forester, it's interesting to see the way forests interact, it's cool to witness," says Grotta. "But for people living in areas covered in caterpillars, it's a nuisance, and they are ready to be done."